Lumber which has recently been cut and machined contains a relatively large percentage of water and is referred to as green lumber. Prior to being used in construction or other applications which demand selected grades of lumber, the green lumber must be dried to remove a relatively large percentage of water. Acceptable water content will vary with the application as well as the type of wood, however, in many circumstances, a moisture content of 19% or less is acceptable.
Although lumber may be air dried, kiln drying accelerates and provides increased control over the drying process. In kiln drying, a charge of lumber is placed in a kiln chamber. A typical kiln chamber is a generally rectangular building which can be sealed to control the introduction and exhaust of air. Further, such kiln chambers typically have reversible fans for circulating the air through the chamber.
The charge of lumber placed in the kiln generally consists of a number of rectangular solid stacks of lumber. Each stack of lumber, in turn, typically consists of a number of vertically stacked, horizontal rows of lumber that form a rectangular solid. The horizontal rows are spaced apart for air to pass between the rows using wooden boards referred to as "stickers" that have a relatively small lateral cross-sectional area in relation to the lateral cross-sectional area of the lumber forming the charge. The stickers are generally spaced apart between each horizontal row to allow air to flow between the rows.
Typically, the stacks of lumber are placed on separate wheeled, flat bed cars which are mounted for movement on railroad-type tracks. Kilns may have any desired number of tracks. Multi-track kilns may therefore accept several stacks of lumber during each drying cycle.
In operation, a charge of green lumber is initially placed in a kiln chamber. After sealing the kiln chamber, the air within the kiln is heated to facilitate drying. The air may be heated in a number of ways such as by heat transfer from pipes extending through the kiln chamber in which steam flows. Alternatively, heated air may be introduced such as from a furnace. Kilns which utilize the introduction of heated air are typically referred to as direct fired kilns.
Fans are generally positioned in upper portions of the kiln and above the stacked lumber to circulate the heated air through the kiln chamber, including the stacks of lumber. Because the stickers provide spacing between the horizontal rows of lumber, the heated air passes between the rows of lumber and is in direct contact with both the upper and lower surfaces of the individual pieces of lumber. The fans continually recirculate the air through the kiln and the lumber to further dry the lumber. Periodically, a portion of the circulating air is exhausted from the kiln and additional air is introduced into the kiln. The additional air is typically heated in the kiln chamber, such as by heat transfer from the steam pipes. This periodic exhaust and replacement process allows circulating air which has absorbed a large amount of moisture from the green lumber to be removed, while drier air is introduced to accelerate the lumber's drying.
Within such kilns, the circulating air flows in a generally circular pattern. More particularly, fans above the stacks of lumber direct air laterally over the top of the lumber in a first direction. When the air contacts a first sidewall of the kiln chamber, the bulk of the air is forced downward by the fans and the ceiling of the kiln chamber. The circulating air subsequently flows through the spaces between the horizontal rows of lumber established by the stickers in a second, lateral direction opposite the first lateral direction. Upon contact with a second sidewall of the kiln chamber, the air rises and is recirculated by the fans through the lumber. Periodically, the fans are reversed such that the air flows in the opposite direction to provide generally consistent drying of the lumber.
Kiln chambers are generally rectangular in lateral cross-section. In addition, the fans located in upper portions of the kiln chamber each include a plurality of fan blades having an axis of rotation. The axis of rotation of each is fan is typically substantially perpendicular to the generally vertical sidewalls of the kiln chamber. Consequently, the air circulated by the fans flows in a direction substantially perpendicular to the sidewalls of the kiln chamber. Thus, instead of circulating downward along a sidewall of the kiln chamber and then through a stack of lumber, a portion of the air circulated by the fans is deflected from the sidewalls. The deflected air typically remains in upper portions of the kiln chamber and does not pass over the surface of the lumber thus reducing the extent to which the circulating air draws moisture from the green lumber.
In order to improve airflow, splitter plates have been incorporated in kiln chambers to direct the flow pattern of air circulated by the fans downwardly about the stacks of lumber. Typical splitter plates extend longitudinally throughout the length of the kiln chamber and laterally from a leading edge in upper portions of the kiln chamber downwardly toward a sidewall of the kiln chamber to a trailing edge. The leading edge of conventional splitter plates is laterally spaced from the fan, but is positioned so as to receive and direct at least a portion of the air circulated by the fans.
Typical splitter plates also have a curved portion between the leading edge and the trailing edge for directing the circulating air downward about the stacks of lumber in the kiln chamber. The curved portion of conventional splitter plate has upper and lower surfaces over which air is passed. The air directed along the lower surface is generally guided along the splitter plate due, at least in part, to the upward movement of the heated air against the lower surface of the splitter plate. This air is also guided along the lower surface of conventional splitter plates by the generally upward movement of the swirling air circulated by the fans at the time the air initially contacts the lower surface.
The portion of circulating air which contacts and is to be guided by the upper surface of conventional splitter plates, however, does not follow the contour of the upper surface of the splitter plate. Instead, through a phenomenon termed "separation" a portion of the circulating air flowing along the upper surface separates from the splitter plate and remains in upper portions of the kiln chamber. Separation produces gross inefficiencies within any fluid system, including a kiln drying system. Thus, although conventional splitter plates improve the downward circulation of air about the rectangular stacks of lumber, a portion of the air circulated by the fans separates from a splitter plate and remains in upper portions of the kiln chamber so as to not flow across the surface of the lumber.
Accordingly, for a given period of time, less lumber can be dried in a kiln chamber in which a portion of the air circulated by the fans separates from a splitter plate and remains in upper portions of the kiln chamber than in a kiln chamber in which all of the circulating air flows through the stacks of lumber. Alternatively, the same amount of lumber may be dried, but in a longer length of time in a kiln chamber in which a portion of the air circulated by the fans separates from a splitter plate and remains in upper portions of the kiln chamber than in a kiln chamber in which all of the circulating air flows over the surface of the lumber. Since it is expensive and time consuming to heat the air to dry the lumber, the resultant increase in drying time and the decrease in drying capacity of the kiln chamber significantly increases the expense of kiln drying.